1. Field of the Invention
This invention relates generally to the reproduction of sound in multichannel systems generically known as “surround-sound” systems and more specifically to the application of psychoacoustic principles in the design of a loudspeaker system for reproducing a surround sound experience from loudspeakers located only in front of the listener.
2. Background Art
It has long been recognized that it is possible to use interaural crosstalk cancellation (IACC) and head related transfer functions (HRTF) to expand the perceived soundstage of a two channel audio system or to create the illusion of sounds coming from phantom locations independent of the actual location of the loudspeakers. Through the 1970's and 1980's a number of audio components were available for purchase which used IACC to expand the perceived soundstage. However, until the availability of inexpensive, powerful digital signal processing (DSP) more accurate generation of phantom sound sources at specific locations was very difficult and costly due to the complexity of accurate HRTF synthesis.
More recently the availability of DSP and improved filtering algorithms has made it possible to create a phantom sound source in almost any location using just a single pair of loudspeakers typically located in front of the listener. Using variations of the same techniques it is possible to create several phantom sound sources at the same time from a single pair of loudspeakers typically located in front of the listener. This technique has many practical applications. For example, the experience of having front, rear and center speakers as in a complete 5.1 surround sound audio system can be simulated using a single pair of loudspeakers or headphones.
These techniques are based on the way in which human beings process sounds received by their ears to determine the location of the sources of those sounds. In general, we hear the direction of sounds based on two primary mechanisms, Interaural Time Delays (ITD) and Interaural Level Differences (ILD). ITD refers to the additional time required for a sound located to one side of the listeners head to arrive at the opposite side ear as compared to the time required to reach the near side ear. The ITD of a sound allows the listener to determine the lateral direction of a sound with great precision. ILD refers to the difference in perceived intensity between the listeners two ears for a sound arriving from a particular location. For example, a sound located to the listeners left would appear generally louder in the left ear as compared to the right car due to a reduction in loudness as the sound passes across the listener's head. Overall intensity differences between the ear reinforce lateral localization of sounds through ITD's. In addition, sounds arriving from a particular direction produce a complicated frequency response pattern at each ear which is characteristic of that specific directional location. The combination of these characteristic directional frequency response curves and the ITD's associated with sounds
arriving from that direction are referred to as Head Related Transfer Functions (HRTF). The frequency response component of the HRTF's is quite complex and somewhat different for each individual. It is the detailed structure of the HRTF frequency response at each ear that allows the listener to determine the elevation of a sound and whether it is in front or behind. For example, a sound source located 60 degrees to the left and in front of the listener has the same ITD (approx. 300 ms) as a sound source located at 60 degrees left and behind the listener. However, the asymmetry of the outer ear produces very different HRTF's for those two sound source locations thereby allowing the listener todetermine both the lateral location and front versus back. A similar mechanism allows the listener to determine the approximate elevation of a sound source. In general the mechanism for determining lateral location of sounds based on ITD's operates in the frequency range of approximately 150 Hz to 1,200 Hz. The mechanisms for localizing sounds based on the frequency response of HRTF's operates from approximately 500 Hz to above 12,000 Hz.
Based on these principles various methods have been devised for canceling interaural crosstalk in loudspeakers, generating phantom sound sources from monaural signals using synthetic or measured HRTF's and for using HRTF's to create phantom rear channels for an audio surround sound system from only a front pair of speakers.
In general, methods using HRTF's to create phantom sound sources, whether for simulation of a surround sound audio system or other application, have a number of practical limitations. Accurate representation of HRTF's is very computation intensive and it is therefore difficult to obtain sufficient accuracy using practical and cost efficient DSP methods. For example, U.S. Pat. No. 6,173,061, which describes a method for phantom sound source generation using HRTF's, acknowledges the need for more efficient sound processing algorithms and seeks to address this problem. Additionally, the specific HRTF's used in prior art methods are selected on the basis of assumptions regarding the characteristics of the loudspeakers employed, the specific positional relationship between the loudspeakers and the listener, and the variation of actual HRTF's from listener to listener. Given the highly specific and detailed nature of HRTF's, those skilled in the art will recognize that changes in the loudspeaker characteristics or locations combined with movement of the listener away from the assumed listening location can easily destroy the phantom sound source illusion. Also, the actual HRTF's of some listeners may be too different from the HRTF's employed
in the device for the illusion to work. For example, U.S. Pat. No. 4,893,342 and its related patents describe methods for increasing the positional flexibility of an HRTF based method by limiting the frequency range of the HRTF representations to a range of approximately 600 Hz to 10 kHz and methods for determining listener tolerant HRTF's.
Some known methods for creating phantom sound locations and sources rely on the use of binaurally recorded signals or other specially recorded signals as inputs. These methods may be subject to the above described limitations and will also function properly only when using input signals made with the specified recording scheme. For example, U.S. Pat. No. 4,199,658 describes such a method based on the use of binaurally recorded signals as inputs.
Finally, most known methods for creating phantom rear channel sound sources seek to reproduce the illusion that actual loudspeakers are located at specific locations behind the listener. Such methods are disclosed, for example, in U.S. Pat. No. 6,052,470 and its related patents which describe various methods for using HRTF's to create the illusion of a pair of speakers located behind the listener. However, those skilled in the art generally agree that in rear channel sound reproduction for an audio surround sound system, diffuse localization is preferable to the type of specific localization provided by actual rear located direct radiator loudspeakers. Furthermore, as will be understood by those skilled in the art, audio surround sound systems composed of front and rear pairs of speakers are not effective in localizing sounds in the general areas
directly to the left and right of a listener located centrally between the two pairs of speakers.
Therefore, there exists a need for methods for creating phantom rear surround sound channels which require less complicated signal processing, which are more tolerant of loudspeaker characteristics, loudspeaker placement, listener location and listener to listener HRTF variations, which are effective when using commonly available recordings and which are capable of diffuse localization of rear channel sounds in an audio surround sound system over a range of locations around the listener.